1. Technical Field
The present disclosure relates to an optical phase shifting plate, and more particularly, to an optical phase shifting plate in which a heater is bonded to an optical substrate that can change the refractive index for transmitted light by the thermooptical effect and which adjusts the optical path length of the transmitted light by a thermal response of the optical substrate.
2. Related Art
An optical phase shifting plate is configured to finely adjust the optical path length in a space optical system, and used as phase shifting means for a Michelson interferometer, a Mach-Zehnder interferometer, or the like. A demodulator including a Michelson interferometer or a Mach-Zehnder interferometer is disclosed in JP-A-2007-151026, for example.
FIG. 5 is a functional block diagram describing the operation of an optical phase shifting plate used for a Michelson interferometer. The Michelson interferometer 500 includes a beam splitter 501 and planar mirrors 502, 503.
The beam splitter 501 is a planar member in which a dielectric multilayer film 501b is formed on a glass substrate 501a. The beam splitter 501 reflects an incident light beam L500 which enters at a certain angle (for example, 45°), and allows the incident light beam L500 to transmit therethrough to split the light beam L500 into split light beams L501, L502 having a certain intensity ratio (1:1).
The beam splitter 501 multiplexes the split light beams L501, L502 which are reflected respectively from the planar mirrors 502, 503 such that the split light beams L501, L502 interfere with each other, and then splits the interference light beam which is obtained as a result of the interference, at a certain intensity ratio (1:1).
According to the configuration, the planar mirrors 502, 503 are positioned so that the optical path length of the split light beam L501 is longer by a predetermined length than that of the split light beam L502. Thus, the split light beam L501 is delayed by a predetermined time from the split light beam L502.
Thereafter, the split light beams are multiplexed by the beam splitter 501 to interfere with each other, whereby the phases of the split light beam L502 and the split light beam L501, which is delayed by the predetermined time, are compared with each other. Interference light beams having an intensity which corresponds to a result of the phase comparison are output as output light beams L503, L504.
An optical phase shifting plate is provided between the beam splitter 501 and the planar mirror 502 to compensate time delay caused by temperature variation, and finely adjusts the optical path length of the split light beam L501 (i.e., the delay time) thereby performing the phase adjustment of the optical path of the space optical system.
Also, the optical phase shifting plate includes: an optical substrate 1 made of silicon which can change the refractive index by means of the thermooptical effect; and a chip heater 2 bonded onto the optical substrate 1. The chip heater 2 is heated by a control current I supplied from phase adjusting means 3, and the optical path length of the split light beam L501 which passes through the optical substrate 1 is adjusted by the thermal response of the optical substrate 1.
FIG. 6 is a perspective view showing an optical phase shifting plate using the chip heater according to the related art. The chip heater 2 is commercially available as a chip component, and is adhesively fixed to the optical substrate 1 in a certain region that is away from a region through which light beam passes.
FIG. 7 is a side view showing an optical system using the optical phase shifting plate according to the related art. The optical substrate 1 is attached to an attaching portion 4a provided at the bottom portion of a package 4. Optical components 5, 6 such as a beam splitter and a planar mirror are provided on the package 4 with the optical substrate 1 interposed therebetween. The optical substrate 1 is placed in the middle of an optical path L between the optical components so as to be perpendicular to the optical path.
The chip heater 2 is bonded to the optical substrate 1, and connected to a current input terminal 8 which is provided at a side portion 4b of the package 4, through a wire such as a thin copper wire. The wire is provided in a hollow portion of the package 4, and the heating current I is supplied to the chip heater 2 through the wire.
The structure of the related art optical phase shifting plate has the following problems.
(1) An electric power must be supplied to the chip heater through the wiring such as a thin copper wire which is passed through the air. In order to perform the wiring connection without interrupting the light beam path, a space for the wiring must be ensured in the package, and a wire fixing mechanism for preventing the wiring from vibrating (e.g., a projection) is required to for ensure the reliability. However, such a structure causes the problems that the package structure becomes complicated and the production cost is increased.
(2) When the chip heater is used, the optical substrate needs to be enlarged in accordance with the volume for disposing the heater. Moreover, the thermal capacity is increased as the volume of the chip heater is increased. Thus, the responsibility of the phase shift is deteriorated, and also the power consumption is increased.